1. Field of the Invention
The present invention relates to a reciprocating engine of a spark ignition type and, more particularly, to a spark ignition type reciprocating engine of a natural intake system with ignition plugs disposed so as to face combustion chambers of the engine.
2. Description of the Related Art
Among engines of a natural intake system, i.e. internal combustion engines without superchargers, spark ignition type reciprocating engines are loaded on many automobiles, each of which is provided with ignition plugs so as to face the combustion chambers defined and delimited by the pistons inserted into the cylinders so as to be movable in a reciprocating way. It is well known that the engines of this type can provide better heat efficiency as a compression ratio becomes larger.
It can be noted, however, that there is the tendency that knocking becomes likely to occur when a larger compression ratio is set. For conventional engines except for engines so adapted as to be suitable for use with a so-called high-octane gasoline having a large octane value, a compression ratio (.epsilon.) is set at largest to .epsilon.=about 10.